This section provides background information related to the present disclosure which is not necessarily prior art.
VISCTRONIC® fan drives that are commercially manufactured by BorgWarner Inc. of Auburn Hills, Mich., are examples of devices that employ shear forces on a working fluid to transmit rotary power. In such devices, a relatively high viscosity working fluid, such as a silicone fluid, is transmitted into a working cavity between a disk and an outer housing assembly. The disk is coupled to an input member for rotation therewith, while the outer housing assembly can be coupled to a fan for common rotation. The input member that drives the disk can be driven by a belt of a front engine accessory drive that is driven by an engine's crankshaft. The disk and the outer housing assembly cooperate to form a flow path that is configured to generate shear forces in the working fluid that in turn creates torque that drives (i.e., rotates) the outer housing assembly. The generation of shear forces in the working fluid, particularly when relatively high levels of torque are desired, generates heat in the working fluid.
To aid in rejecting heat from these devices, the outer housing, which is commonly formed of aluminum, can be formed with a plurality of cooling fins. The cooling fins effectively increase the surface area of the exterior surface of the outer housing assembly and increase the ability of these devices to reject heat to the atmosphere via conduction, convection and radiation. The cooling fins, however, do nothing to promote heat transfer from the working fluid to the outer housing assembly.
The heat that is generated when the output housing assembly slips relative to the input member is commonly called “slip heat”. The magnitude of “slip heat” generated at a given operational condition is equivalent to the product of the fan torque at that condition and the associated “slip speed” (i.e., the rotational speed differential between input and output members). “Slip heat” is therefore minimal at the extreme conditions of 0% slip and 100% slip. In between these limits, in the region where output to input speed ratio is around 50% to 60%, “slip heat” is generated at its maximum rate. For this worst-case “slip heat” condition, only a small portion of the available working fluid is present in the working cavity; a majority of this smaller portion of fluid resides in the region adjacent the OD of the rotor (disk). This creates a particularly difficult problem to overcome; high “slip heat” magnitude is entering into a relatively small volume of fluid that has a relatively small wetted surface in contact with the walls of the output housing. This problem has been present with all viscous fan drives since the beginning of their usage in automotive engine cooling circa 1950's-1960's.
We understand that a person of ordinary skill in the art would have assumed that “slip heat” is an inherent problem and that the above-described worst case “slip heat” condition simply must be designed around, since the typical fluid shear gap between input and output surfaces is generally very small (approximately 0.4 mm), and it has not been conceivable that high thermal gradients could exist in that tiny shearing region. Recent advances in fluid material understanding have become possible through the utilization of CFD (Computational Fluid Dynamics). In an effort to understand how to optimize our invention to a given viscous fan clutch, we investigated the thermal gradients that exist in the thin fluid shear zone between the disk and the outer housing assembly (which are typically rotating at different rotational speeds). Our investigations of the thermal gradients that exist in the thin fluid shear zone have revealed that completely laminar shear layers are set up that do not effectively transport thermal energy from layer to layer. Furthermore, we observed that the gradient distribution tends to be very non-linear, which we believe to be caused by the non-Newtonian nature of the silicone working fluid that thins with both temperature and shear-rate. We observed this non-linearity to cause the boundary layer adjacent the colder walls of the output housing to be exceptionally thick and thermally insulative.
U.S. Pat. No. 5,577,555 discloses a heat exchanger having a stationary tube that is configured to transmit an aqueous solution (e.g., “a lithium bromide aqueous solution including a surface activating agent”). The tube defines a heat exchange wall having a plurality of “dents” formed therein. The “dents” are described as having a depth that is larger than a thickness of the tube wall and between 0.6-2.0 mm. The size of the tube is not disclosed, but a flow rate of the aqueous solution flowing through the tube is “preferably 0.7-0.25 kg/(m×s)”. While the '555 patent does not describe the effect that the “dents” have on the aqueous solution that flows through the tube, it appears to us that the “dents” induce a transition from laminar flow to turbulent flow in a portion of the flow of the aqueous solution that is near the wall of the tube. If an aqueous solution of lithium bromide is assumed to have a density of 1500 kg/m3, a dynamic viscosity of 0.006 Pa·sec, and a mass flowrate of 0.475 kg/sec, and the tube diameter is assumed to be 25 mm, the average flow velocity would be 0.645 m/s. The corresponding Reynolds number is 4031.
In fluid mechanics, a dimensionless quantity known as a Reynolds number is employed to predict flow patterns. The Reynolds number is a ratio of inertial forces to viscous forces and can be calculated by the following formula:Re=(V·L)/v where Re is the Reynolds number, V is the fluid velocity, L is a characteristic length, and v is the kinematic viscosity of the fluid. In a pipe, laminar flow is associated with a Reynolds number that is less than about 2000, turbulent flow is associated with a Reynolds number that is greater than about 4000.
Accordingly, inducement of the transition from laminar flow to turbulent flow in the tube disclosed in the '555 patent appears to be possible due to a relatively high velocity of the aqueous solution (which helps to provide a relatively large numerator in the formula for calculating the Reynolds number) and a relatively low kinematic viscosity of the aqueous solution (which provides a relatively small denominator in the formula for calculating the Reynolds number).
In contrast, the working fluid in the above described fan drives is highly viscous (i.e., the parameter v in the denominator of the formula for the Reynolds number is relatively large). As such, the denominator in the formula for the Reynolds number is relatively large so that the resulting Reynolds number is relatively small so that inducement of turbulence is not possible. For example, a fan drive operating at a 50% slip condition with a slip speed of 1500 rpm in which the disk has a disk radius of 118 mm, a radial shear gap between the disk and the outer housing assembly is 1.2 mm, and a kinematic viscosity of the working fluid is 500 cSt at ambient temperature, the resulting Reynolds number is 44.5, which is significantly below a transition to turbulent flow that begins at Reynolds numbers exceeding 2000.
In view of the above remarks, there remains a need in the art for an apparatus that employs shear forces to transmit energy in which the apparatus is better configured to reject heat from a working fluid where turbulent flow mixing of the thermal boundary layers is not a possibility.